The long-term objectives of this research application is to better understand the cellular and molecular basis of the inflammatory response in order to develop better therapies to augment it in conditions where the host's immune response is compromised, and to inhibit it under conditions where the response is counterproductive, such as in inflammatory diseases like atherosclerosis, rheumatoid arthritis and other autoimmune diseases, septic shock, asthma, and transplant rejection. A critical step in the inflammatory response is the migration of leukocytes out of the bloodstream to the site of inflammation. We have been studying the molecules and mechanisms responsible for diapedesis-the step in this process in which leukocytes pass across the endothelial cells lining postcapillary venules at sites of leukocyte egress. In the first funding period of this grant, we discovered a molecule (CD99) and a mechanism (targeted recycling of membrane from an intemal perijunctional compartment called the LBRC) that play significant roles in diapedesis. In the second funding period thus far (i.e., the first 3.5 years of the MERIT award) we have already accomplished most of the Specific Aims of the 5 year plane: We have investigated how CD99 regulates diapedesis. We have discovered that it is present in the LBRC and its function requires the presence of PECAM in the compartment as well. We have cloned the mouse version of CD99 and the related molecule CD99L2 and demonstrated that both play a role in transmigration. We have demonstrated that transcellular migration involves the same mechanism as paracellular migration: Targeted membrane trafficking from the LBRC to the site of migration in a microtubule-dependent manner. The goals for the MERIT extension period are to develop the work proposed for these specific aims to answer the next important questions. These include: What signals from CD99 are required to complete transmigration? Why does CD99 function depend on PECAM? (How do these molecules interact?) Do CD99 and CD99L2 complement each other's function in vivo? If so, how? Is blocking them therapeutic in models of chronic inflammatory disease? How is LBRC membrane recruited for transcellular migration in vitro and in vivo?